Exporting a mesh of specific dimensions
I think the mismatch that is giving you a challenge is between the default sizes of the dimensions. The default size of a unit in Blender is one meter. The default size of a unit of measurement in Meshlab is one centimeter. If you create a blender object that measures 4 blender units in one dimension, and 2 blender units in another, when imported to meshlab, it will be 4 centimeters in one dimension, and two centimeters in the other. 

You either need to account for the difference in sizes when you create your model, or change the units in Blender so that one grid square is one centimeter, instead of one meter.